The invention relates to an X-ray image detector, comprising a plurality of X-ray sensitive sensors and having the following features:
each sensor comprises a collecting electrode and a switching element which connects the collecting electrode to an output lead; PA1 a photoconductor layer is provided between the individual collecting electrodes and a bias electrode; PA1 in conjunction with reference electrodes, the collecting electrodes form capacitances which can be charged by charge carriers generated in the photoconductor. PA1 the collecting electrode is arranged adjacent the output lead; PA1 the switching element and the output lead are covered by an insulating layer; PA1 the insulating layer and the collecting electrode are covered by a semiconductor layer; PA1 the semiconductor layer is doped so that for the charge carriers flowing in the direction of the collecting electrode it has a conductivity which is high in comparison with its conductivity for charge carriers of opposite polarity.
X-ray image detectors of this kind are used inter alia for fluorosscopy where X-ray images are made in a close succession in time, for example 60 images/second. In particular, FIG. 1 shows a circuit diagram of an X-ray sensor matrix and FIG. 2 shows a thin-film structure of such an X-ray sensor matrix in a plan view (FIG. 2a) and a cross-sectional view (FIG. 2b). A circuit diagram of such an X-ray image detector being an X-ray sensor matrix which is known from EP-OS 444 720 is diagrammatically shown in FIG. 1. For each pixel there is provided a sensor which comprises a switching element 1, a capacitance 2 as well as a photosensor. The typically 2000.times.2000 switching elements are realized on a common dielectric substrate (glass) using a thin-film technique (for example, in the form of thin-film field effect transistors 1 as shown in FIG. 1). The photosensors are formed by a continuous photoconductor layer 3 covering the entire image area, a bias electrode 4 provided on the photoconductor layer, and for each pixel a separate collecting electrode 11 which is provided on the other side of the photoconductor layer. When the image detector is struck by X-rays in the operating condition, charge carriers are generated in the photoconductor layer 3, said charge carriers flowing through the photoconductor to the collecting electrodes 11 under the influence of an electric field generated by means of a bias source 40 connected to the bias electrode 4. The capacitances 2 connected to the collecting 11 electrodes are thus charged, the other electrode of said capacitance being connected to an electrode 10 carrying a reference potential.
The sensors are arranged in rows and columns as in a matrix, the spacing between the rows and the columns being the same. This spacing determines the spatial resolution. The gate electrodes of the thin-film field effect transistors constituting the switching elements are row-wise interconnected by means of a drive circuit 6 for the purpose of reading. To this end, the gate electrodes of all switching elements of a row are connected to a respective common switching lead 5. The source electrodes of the thin-film field effect transistors are connected to the respective, associated capacitance, whereas their drain electrodes are column-wise connected to a common output lead 7.
The FIGS. 2a and 2b are a plan view and a cross-sectional view, respectively, of a part of the X-ray image sensor matrix, wherefrom the photoconductor layer 3 and the bias electrode 4 have been omitted. Even though the representation is substantially simplified, it still shows the essential elements. On a substrate 15 there are provided a reference electrode 10 (preferably connected to ground potential) as well as the switching leads 5. The switching leads 5 comprise perpendicularly extending tappings 17 which form the gate electrodes of the thin-film field effect transistors. Above the gate electrode 17 there is provided a multilayer structure 12 consisting of semiconductor and insulating layers which constitutes, in conjunction with the electrodes 7 (drain) and 11 (source) a thin-film field effect transistor. The electrode 11 thus serves as the collecting electrode and the source electrode and also constitutes the storage capacitance 2 in conjunction with a reference electrode 10, covered by this electrode, and an intermediate dielectric.
In the thin-film arrangement shown in FIG. 2, the layers between the electrodes are very thin (of the order of magnitude of from 0.2-1 .mu.m). Therefore, it is important that the collecting electrodes do not cover the switching leads, but notably not the output leads 7, because high parasitic capacitances would then be formed between the collecting electrode 11 and the relevant electrodes. In the case of the output leads 7, this would lead to capacitive signal coupling-out and hence to a reduction of the signal to be read, and also to increased noise of the output amplifier 8 connected to the output leads 7, because these amplifiers "see" a higher input capacitance. The collecting electrodes 11 on the one side and the leads 5, 7 on the other side, therefore, must be adjacently arranged, i.e. in the plan view of FIG. 2a the collecting electrodes may not overlap the leads 5 and 7. Thus, they must be restricted to the surface area remaining between two neighboring switching leads 5 or two neighboring output leads 7.
Because all leads 5, 7 and 10 should have a width of between 10 and 25 .mu.m in order to achieve adequate conductivity, the portion of the overall surface area of an X-ray image detector which is occupied by the collecting electrode will be smaller than the pixels, or their spacing is smaller, i.e. as the resolution is higher.